Separation and sensitive determination of metal ions by capillary zone electrophoresis with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol

Abstract

Capillary electrophoresis of 22 metal ions was examined with a visible light-absorbing chelating agent, 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (5-Br-PAPS). The metal chelates separated in a capillary were measured at 550 nm by on-column visible absorption detection. In a standard procedure, carriers (electrophoretic buffers) containing the chelating agent and pH buffer components were used, and sample solutions containing excess amounts of the chelating agent were injected using a vacuum injection method (about 1 × 10^–8 dm³). Fourteen metal ions were detected at pH 8.7, and the apparent electrophoretic mobilities (–µ_ep′, 10^–4 cm V^–1 s^–1) were as follows: Ag⁺, –4.21; Co²⁺(Co³⁺), 1.37; V^IV, 1.74; Pd²⁺, 2.36; Mn²⁺ and Hg²⁺, 2.43; Ni²⁺, 2.48; Fe²⁺, Fe³⁺ and Cu²⁺, 2.50; Cd²⁺, 2.55; Zn²⁺, 2.58; Pb²⁺, 2.62; and Ti^IV, 3.27. By lowering the pH of the carrier, the separation of metal ions was improved, and the apparent electrophoretic mobilities at pH 4.9 were as follows: Ag⁺, –6.50; Cd²⁺, –3.78; Zn²⁺, –2.78; Pb²⁺, –2.21; V^IV, 0.17; Mn²⁺, 0.48; Hg²⁺, 0.57; Cu²⁺, 0.60; Co²⁺(Co³⁺), 1.37; V^V, 2.01; Ni²⁺, 2.34; Fe²⁺, 2.47; and Pd²⁺, 2.55. At pH 4.9, these metal ions were separated within 7 min by using a fused-silica capillary (50 µm i.d.) of a 50 cm effective length at an applied voltage of 30 kV. The detection limits obtained by the standard procedure at pH 4.9 were as follows (mol dm^–3): Fe²⁺ and Co²⁺(Co³⁺), 5 × 10^–8; Cu²⁺, 3 × 10^–7; Zn²⁺, Pb²⁺ and V^IV, 5 × 10^–7; and Cd²⁺, 1 × 10^–6. The method was applied to the determination of trace amounts of cobalt contained as impurities in nickel salts.